An X-ray phase contrast technique is a technique for acquiring a phase image of an object by detecting a phase shift of an X-ray caused by the object.
One of phase contrast techniques, scanning double-grating (hereinafter referred to as SDG) as disclosed in NPL1 has been devised in recent years. SDG uses a diffraction grating which forms a periodic interference intensity distribution (hereinafter referred to as an interference pattern) at a specific distance from the diffraction grating and a shielding grating which is placed at the position where the interference pattern is formed. Detecting an X-ray intensity distribution while simultaneously moving the two gratings enables acquisition of a phase contrast image in one imaging operation.
FIG. 7 illustrates a configuration of an imaging apparatus which performs SDG described above.
The imaging apparatus illustrated in FIG. 7 includes an X-ray source 111 which has spatial coherence on a diffraction grating such that an interference pattern is formed and a diffraction grating 113 for dividing an X-ray applied from the X-ray source 111 into diffracted light components and forming an interference pattern. The imaging apparatus is also provided with a shielding grating 114 including periodically arranged shielding portions blocking an X-ray and transmitting portions transmitting an X-ray, which is placed at a position where an interference pattern is to be formed. The imaging apparatus further includes a detector 115 which detects an intensity distribution of an X-ray transmitted through the shielding grating, a moving unit 116 of the diffraction grating, and a moving unit 117 of the shielding grating. Note that the shielding grating 114 has the same period and the same period direction as the period and the period direction of an interference pattern formed by the diffraction grating 113 when an object is not placed between the X-ray source and the position where the interference pattern is formed.
The principle of SDG will be briefly described. When an X-ray emitted from the X-ray source 111 passes through an object 112, the wavefront of the X-ray has a tilt (hereinafter referred to as a differential phase) corresponding to the refractive index and the shape of the object. When the X-ray having passed through the object is diffracted by the diffraction grating 113, an interference pattern with a distortion corresponding to the differential phase of the object is formed. The process of placing the shielding grating 114 at a position where the interference pattern is formed (hereinafter referred to as a Talbot position) and detecting the X-ray transmitted through the shielding grating 114 by the detector 115 enables detection of distortion in the interference pattern caused by the object as an X-ray intensity distribution. However, since the X-ray is periodically shielded by the shielding grating 114, this process can acquire only part of information on the phase of the object.
Accordingly, the X-ray intensity distribution is detected by moving the diffraction grating 113 and the shielding grating 114 in sync with each other in a direction of the period of the shielding grating by an amount corresponding to one period or more while keeping the relative position between the shielding grating 114 and the interference pattern.
The SDG operation enables acquisition of an image in which an absorption image and a differential phase image of an object are superimposed in one imaging operation.
An X-ray intensity distribution acquired by the method of NPL1, however, includes unnecessary components caused by an X-ray transmittance distribution of an object and unevenness in X-ray irradiation by an X-ray source. Accordingly, the method of NPL1 suffers from the inability to accurately measure the phase distribution of an X-ray having passed through an object.